COMMON: Move definition of math constants to scummsys header.

This should have no effect on common/math.h as scummsys is included at
the top anyway, but this will fix compilation on some toolchains when
strict ANSI language compliance is enabled i.e. -std=c++11, rather than
-std=gnu11.

This causes an issue as a side effect is to set a guard which disables
various non-standard defines in the system headers i.e. the M_PI and
M_SQRT1_2 constant definitions in the MinGW x86_64 toolchain on
buildbot.

By moving this into scummsys.h after the point of system math.h
inclusion, we can ensure that M_PI etc. are always present.
COMMON: Add comment explaining reason for defining math constants.
COMMON: Define all Unix98/BSD math constants in scummsys.h.

This should fix compilation on mingw64 with --enable-c++11 and also make sure
the constants are present on other systems for the uncommon case they might be
missing.